Project Summary The Under-5 Mortality Rate (U5MR) is a key and widely-used indicator of child health, but it conceals important information about how this mortality is distributed by age. For better understanding and monitoring of child health, it is critical to examine how the risk of death varies within the 0-5 age range. This includes age breakdowns beyond the standard cut-off points of 28 days (for neonatal mortality) and 1 year (for infant mortality). In many populations, however, the age pattern of under-5 mortality is not well known. Less- developed countries, in particular, lack the high-quality detailed vital registration information necessary for the analysis of such age patterns. Sample surveys collecting retrospective birth histories do not satisfactorily fill this gap, because they are subject to systematic biases that are particularly consequential for estimating age patterns. This makes the need for high-quality information on age patterns of under-5 mortality even more critical, because regularities in these age patterns can be used as a powerful tool for evaluating and correcting data when sources are deficient. The goal of this project is to improve our understanding of age patterns of under-5 mortality by gathering the largest database to date on high-quality global mortality information by detailed age (by days, weeks, months, and years of age) from birth until age 5, by sex. This database, which will cover a wide array of historical and contemporary contexts in both more- and less-developed settings, will serve as a basis for generating models summarizing regularities about how under-5 mortality is distributed by detailed age in human populations. These models will then be used for evaluating and correcting under-5 mortality information by detailed age in less-developed countries. This global database, and models derived from it, will also allow us to address specific substantive questions about how and why age patterns of under-5 mortality vary by sex, time, and place, with important programmatic implications. This database will be made publicly-available, allowing the research community to easily access high-quality primary information on under-5 mortality by detailed age for their own research needs, and for further validation and replication of the project's results.